Published on Web 12/16/2004
Aqueous Suspension Polymerization of Isobutene Initiated by 1,2-C6F4[B(C6F5)2]2 Stewart P. Lewis,† Lee D. Henderson,‡ Brett D. Chandler,‡ Masood Parvez,‡ Warren E. Piers,*,‡ and Scott Collins*,† Department of Polymer Science, UniVersity of Akron, Akron, Ohio 44325-3909, and Department of Chemistry, UniVersity of Calgary, Calgary, AB Canada Received September 9, 2004; E-mail:
[email protected];
[email protected] Aqueous suspension or emulsion polymerization processes are of practical utility, combining high conversion with control over latex morphology.1 In recent years, the aqueous suspension or emulsion polymerization of common alkenes such as ethylene, using transition-metal catalysts,2 and styrene, using cationic initiators,3 has been reported. However, early attempts at the polymerization of isobutene (IB) in aqueous suspension yielded only dimers and trimers,4 thus leading to the general belief that cationic polymerization of this monomer in the presence of significant quantities of water is unfeasible.5 We report here the first polymerization of IB and synthesis of butyl rubber (IIR) in aqueous suspension using diborane 1,2-C6F4[B(C6F5)2]2 (1). Recently, we reported on the use of diborane 1 (Scheme 1) as an initiator of IB polymerization, in combination with either cumyl chloride or adventitious moisture, in hydrocarbon media.6 Further work has revealed that diborane 1 is effective for protic initiation at [1] ≈ 10-5 M in hexane.7 In view of the heightened reactivity of 1 in the presence of water, the reaction of 1 with MeOH was investigated by NMR spectroscopy to identify the products formed. The addition of 0.5 equiv of MeOH to 1 in CD2Cl2 solution at -80 °C leads to formation of an ion pair 2 featuring the [1(µOMe)] counteranion,8 partnered with an oxonium ion, which, based on the 1H NMR spectrum,7 corresponds to [(MeOH)2H] (Scheme 1). On warming above -60 °C this ion pair decomposes, and all of the added MeOH is consumed, forming equimolar amounts of borinic ester 3 (independently prepared from (C6F5)2BCl and MeOH)7 and borane 4, which was identified from its 1H and 19F NMR spectra (Figure 1a).7 Evidently, 2 is susceptible to chemoselective protonolysis of the B-C bonds of the o-C6F4 moiety. In contrast, ion pair 2 is stable in the presence of excess MeOH, even at 25 °C (Figure 1b), a feature that we attribute to the reduced acidity of the oxonium acid, as the solvation of the proton by MeOH is increased.9 Single crystals of the ion pair [(MeOH)3H][1(µ-OMe)] could be grown from a solution of 1 in toluene and MeOH, and the X-ray structure appears in Scheme 1.10 The anion of this ion pair is analogous to that found in [(Et2O)2H][o-C6F4{B(C6F5)2}2(µ-OMe)],11 but features a µ-OMe group that is planar at O1 with essentially equivalent B-O bond lengths of 1.558(3) and 1.566(3) Å, respectively. The endocyclic C-B-O angles at the two B atoms are reduced from the tetrahedral value to 97.8(2) and 97.6(2)°, reflecting angular strain within the fivemembered ring, while the B1-O1-B2 angle is 118.0(2)°. The [(MeOH)3H] cation has not been observed in oxonium acids derived from methanol,12 but is similar to the H7O3+ ion found in a number of structurally characterized salts.13 The central MeOH2 moiety of this oxonium acid is disordered with the one electron (and two protons) occupying sites between O2-O3 and O3-O4,7 † ‡
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J. AM. CHEM. SOC. 2005, 127, 46-47
Figure 1. Variable-temperature 19F NMR spectra of a solution of (a) diborane 1 and MeOH (0.5 equiv) and (b) diborane 1 and a 10-fold excess MeOH in CD2Cl2 solution. Signals due to 1 are highlighted in pink, 2 in blue, 3 in green, and 4 in yellow.
while the O2-O3 and O3-O4 separations of 2.465(5) and 2.495(5) Å differ slightly within experimental error. There are six H-bonds or short contacts between the remaining two, ordered MeOH molecules, and F-atoms of different counterions7 that stabilize this monomeric, oxonium ion within the lattice. Because the basicity of water and MeOH are similar,9 we expected that aqueous suspension polymerization of IB should be possible at sufficiently low T. Experiments involving the addition of a hexane solution of 1 to IB suspended in a 68:32 MeOH/H2O solution at -60 °C were encouraging, but only a low yield ( 2σ(I).7 (11) Henderson, L. D.; Piers, W. E.; Irvine, G. J.; McDonald, R. Organometallics 2002, 21, 340-345. (12) (a) Mootz, D.; Steffen, M. Z. Anorg. Allg. Chem. 1981, 482, 193-200. (b) Hursthouse, M. B.; Newton, J.; Page, P. R.; Villax, I. Polyhedron 1988, 7, 2087. (c) Bonadies, J. A.; Kirk, M. L.; Lah, M. S.; Kessissoglou, D. P.; Hatfield, W. E.; Pecoraro, V. L. Inorg. Chem. 1989, 28, 2037. (13) (a) Stasko, D. J.; Perzynski, K. J.; Wasil, M. A. Chem. Commun. 2004, 708-709. (b) Calleja, M.; Mason, S. A.; Prince, P. D.; Steed, J. W.; Wilkinson, C. New J. Chem. 2001, 25, 1475-1478. (c) Calleja, M.; Johnson, K.; Belcher, W. J.; Steed, J. W. Inorg. Chem. 2001, 40, 49784985. (d) Atwood, J. L.; Junk, P. C. J. Coord. Chem. 2000, 51, 379397. (e) Minkwitz, R.; Schneider, S. Z. Anorg. Allg. Chem. 1998, 624, 1989-1993. (f) Lundgren, J. O. Acta Crystallogr., Sect. B 1979, B35, 780-783. (g) Roziere, J.; Williams, J. M. J. Chem. Phys. 1978, 68, 28962901. (14) Akopov, E. K. Zh. Prikl. Khim. 1963, 36, 1916-1919. (15) Control experiments in the absence of diborane 1 using these mineral acids failed to consume monomer. (16) Shaffer, T. D.; Ashbaugh, J. R. J. Polym. Sci., Part A: Polym. Chem. 1997, 35, 329-344. (17) Metz, M. V.; Schwartz, D. J.; Stern, C. L.; Nickias, P. N.; Marks, T. J. Angew. Chem., Int. Ed. 2000, 39, 1312. (18) Jutzi, P.; Mu¨ller, C.; Stammler, A.; Stammler, H.-G. Organometallics 2000, 19, 1442-1444.
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